The present invention relates to a wavelength multiplexing transmission method and an optical network in which a plurality of optical signals of different wavelengths are multiplexed and transmitted, and an optical transmission apparatus which can be used for the transmission method and the optical network.
Heretofore, a method as stated below has been known as an expedient for extending a ring network based on wavelength multiplexing technology. It is contained in, for example, Rajiv Ramanswasmi and Kumar N. Sivarajan: “Optical Networks—A Practical Perspective—” published by Morgan Kaufmann Publishers, page 449. A general construction for the method has been as shown in, for example, FIG. 10. 15 in the book. The method is founded on the construction that, in case of realizing a ring network of the type which drops or adds only specific wavelengths, optical signals of wavelengths propagating via a certain node apparatus are outputted at the same wavelengths as the inputted wavelengths. Accordingly, a practicable node apparatus is constructed including a dropping section in which any signals are derived from a wavelength demultiplexing unit located on the input side of the node apparatus and are outputted outside, and an adding section in which optical signals given from outside are connected to a wavelength multiplexing unit located on the output side of the node apparatus. Herein, the certain wavelength signals delivered from the wavelength demultiplexing unit are directly delivered to the wavelength multiplexing unit as the signals having the same wavelengths. Thus, the optical signals which are dropped or added by the apparatus itself are externally outputted or inputted via the dropping section or the adding section. On this occasion, the apparatus does not drop or add any wavelengths for itself, but it transmits the optical signals inputted for other apparatuses, from one side to the other side thereof without changing the wavelengths of the optical signals. Although the network of ring scheme (ring network) is exemplified here, a similar method is known also in a linear network. The “linear network” is a network architecture wherein node apparatuses are arrayed in one row, and wherein optical signals of any wavelengths are dropped or added by the node apparatuses arranged midway.
Next, a node apparatus for such a network will be concretely exemplified. FIG. 1 shows an example of the node apparatus which has a wavelength multiplexing function and which incarnates the dropping and adding of specific wavelengths. A wavelength dropping section includes a first space switching unit 1, a wavelength demultiplexing unit 3 and an interface unit 5 for input wavelength-multiplexed optical signal 7. The wavelength demultiplexing unit 3 demultiplexes the input wavelength-multiplexed optical signal 7 into individual wavelengths (λ1, λ2, λ3, . . . , λN), which are respectively delivered to predetermined transmission lines 9. The first space switching unit 1 drops optical signal of desired specific wavelengths in the input wavelength-multiplexed optical signal 7. The interface unit 5 outputs the dropped input lights as desired dropped optical signals. Thus, the wavelength dropping section demultiplexes the input wavelength-multiplexed optical signal 7 into the individual wavelengths and drops the desired wavelengths so as to output the dropped optical signals 12.
On the other hand, a wavelength adding section includes an interface unit 6, a second space switching unit 2 and a wavelength multiplexing unit 4 for output wavelength-multiplexed optical signal 8. The interface unit 6 outputs optical signals to-be-added 13. Optical signals 10 transmitted from the first space switching unit 1, and the optical signals to be-added 13 transmitted from the interface unit 6 are delivered to predetermined transmission lines 11 via the second space switching unit 2 in accordance with connection route settings for the optical signals respectively having the individual wavelengths. The optical signals 11 of the plurality of wavelengths (λ1, λ2, λ3, . . . , λN) thus delivered are wavelength-multiplexed by the output wavelength multiplexing unit 4, and the resulting optical signal is outputted as the wavelength-multiplexed optical signal 8. Here, each of the first space switching unit 1 and the second space-switching unit 2 is constructed of optical switches etc.
Apart from the above expedient in which the dropping or adding section is constructed of the optical switches, a wavelength multiplexer of drop/add type employing “Fiber Bragg Grating” technology has also been proposed. The drop/add type wavelength multiplexer is illustrated in, for example, FIG. 3. 60 on page 172 of the aforementioned book “Optical Networks—A Practical Perspective—”. The fiber Bragg grating technology is optical filter technology which utilizes periodical refractive index modulation within an optical fiber as is formed when the optical fiber doped with Ge (germanium) is irradiated with the interference fringes of ultraviolet light. The construction of the drop/add type wavelength multiplexer employing the fiber Bragg grating technology is shown in FIG. 2. A light dropping section 20 includes a circulator 26 and a splitter 27. In the circulator 26, light propagating rightwards from left is totally transmitted, whereas light propagating leftwards from right is totally reflected downward to the splitter 27 as viewed in the figure. In fiber Bragg gratings 24, only lights of wavelengths λ1, λ2, λ3 and λ4 in the rightward light are totally reflected leftwards. A light adding section 22 includes a combiner 28 and a coupler 29. In the figure, numeral 7 indicates the input wavelength-multiplexed optical signal, and numeral 8 indicates the output wavelength-multiplexed optical signal.
With the fiber Bragg grating technology, the optical signals of the specific wavelengths are derived by a diffraction grating at an input stage in a state where the wavelengths are multiplexed as they are.
It is common to both the dropping/adding methods stated above that the wavelengths of optical signals which are transmitted remain unchanged.
A network wherein a plurality of node apparatuses of the type dropping and adding optical signals of specific wavelengths are connected, has a difficulty as explained below.
In the network wherein the plurality of apparatuses dropping and adding the optical signals of specific wavelengths are connected in a ring scheme or a linear scheme, a request for connecting an optical channel is generally made by designating any two of the plurality of apparatuses which constitute the whole network. In that case, regarding which of wavelengths is to be used for the connection, a wavelength not used in any zone is selected in accordance with the situation of uses of the wavelengths in all zones. Various algorithms corresponding to the individual aspects of uses have been proposed for the selection. Typical examples of the algorithms are as follows: The first example is a method wherein fixed Nos. denoted by natural numbers are assigned to wavelengths usually applied, and wherein an unused wavelength is selected from the smaller one of the Nos. The second example is a method wherein any wavelength is selected from among unused wavelengths by employing a random number.
In an actual transmission circuit, however, a problem is posed as stated below. In general, requests for channels to be connected are not fully determined at the time of the construction of the network. Accordingly, the optical channels of the transmission circuit are added or deleted in accordance with the requests for channels arising every day, and the settings of the channels need to be altered in correspondence with the addition or deletion of the optical channels.
With the prior-art technique mentioned above, it is required to select the wavelength which is not used in any of all the zones of the channel. Accordingly, when a request for connection has occurred in a certain channel, a wavelength which is not used in any of the zones included from a certain apparatus to another apparatus must be selected. In the nonexistence of such a wavelength, the optical transmission channel requested to be connected cannot be connected in spite of the existence of unused wavelengths in the individual zones.
This state is shown in FIG. 3. The figure exemplifies an optical network in the case where node apparatuses A-E, i.e. five apparatuses 100, 101, 102, 103 and 104 are connected in one row. Letters a, b, c, . . . and h indicate channels which are respectively connected to the node apparatuses. The respectively adjacent apparatuses are connected by a multiplex channel of four wavelengths. That is, the multiplex channel can accommodate, up to, four optical channels. Incidentally, the case of the four wavelengths is mentioned here, but in general, the number of wavelengths is not especially restricted. Besides, although the node apparatuses are connected in one row in the example of FIG. 3, the same holds true even when node apparatuses are connected in a ring shape or in a mesh shape. Further, the algorithm of minimum value selection as is the simplest algorithm shall be adopted here. Also, a request for a channel shall be an additional request in the ensuing description.
It is now assumed that requests for connections have occurred in the order of the channels a, b, c, d, . . . and g. Then, the wavelengths of the smallest Nos. usable in compliance with the requests for the connections of the channels are selected on the basis of the algorithm of the minimum value selection. Thus, the seven channels from the channel a to the channel g are set as shown in FIG. 3. By way of example, the channel a connects the node apparatuses A and B, and the wavelength of wavelength No. 1 is used for the channel. The connections of the other channels are similarly understood. Here, it must be attended to that, in the case of the method which uses the same wavelengths in all the channels connected, each of the node apparatuses outputs the same wavelengths as the inputted wavelengths.
It is now considered that the request for the connection of the channel h has been further added. The channel h corresponds to the request for the connection from the node apparatus B to the node apparatus E. When the situation of uses of the wavelengths in the node apparatus C is viewed here, wavelength Nos. 3 and 4 are already used on the left side (on the side of the node apparatus B), and wavelength Nos. 1 and 2 are already used on the right side (on the side of the node apparatus E). Consequently, any wavelength usable on both the right and left sides in common does not exist in the node apparatus C. Accordingly, this example involves the problem that the channel h cannot be added though the node apparatus although C has the unused wavelengths on both the right and left sides. For adding the channel h, therefore, it is necessary to build, for example, another network of ring scheme constituted by a plurality of similar apparatuses. This means that a wavelength multiplexing capability is not fully exploited in the multiplex system of wavelength multiplexing.